thank you everybody for explaining what's going on!!
And Kostya, thank you for getting involved right away!
I tried switching back and forth and it works for some time. However after a while AquaMail shows the 12h-format again.
Phone is a Galaxy S4 running 5.1.1 (OptimizedCM-12.1-20150725-jflte / LMY48G / Firmware XXUAMDE) from a thread over at xda-developers (title: [JDCTeam][5.1.1][25 July] Optimized CyanogenMod 12.1 builds)
However, since I installed xposed I can't hope for support from their side.
But I suppose he wants to use 12 hour mode in the whole system, just in AquaMail 24-hr mode for replies
Then that's the time to decide, which format he / she really likes best.
Just use h instead of H in your SimpleDateFormat (if that option is enabled)
Didn't mean to confuse - my phone is and was set to 24h and I want to keep it that way.
Kostya, is there anything you can do about it (use h instead of H)?
Or do we have to wait for google (maybe as well custom rom creators?) to fix it?